Method of reducing secondary emission induced by radiation



United States Patent 3,371,146 METHOD OF REDUCING SECONDARY EMISSION INDUCED BY RADIATION Dinstlale M. J. Compton, Del Mar, Califi, assignor by mesne assignments, to the United States of America as represented by the Secretary of the Army N0 Drawing. Filed July 1, 1965, Ser. No. 469,007 1 Claim. (Cl. 174-17) ABSTRACT OF THE DISCLOSURE The reduction of the secondary emission of electrons, caused by exposure to intense high energy radiation, from ordinary components in electronic systems is accomplished by introducing a highly electronegative gas into the electronic system. The preferred gas for this purpose is a mixture of one atmosphere nitrogen dioxide and dinitrogen tetroxide which form a stable mixture in the gaseous state at low to moderate temperatures. The gas may be introduced into the electronic system by any conventional means well known to those skilled in the art.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment to me of any royalty thereon.

This invention relates generally to a method of reducing the nuclear vulnerability of electronic equipment, and more particularly to a method of reducing secondary emission induced by radiation in electronic equipment.

When an electronic system is exposed to intense high energy radiation, for example in the form of a pulse from a reactor or a bomb burst, several effects take place which affect its operation and may give rise to spurious signals. These effects include conductivity induced in insulators, secondary emission from metals and insulators, effects of various kinds in semiconductors, and conductivity in gases. In discussing radiation effects, it is customary to make a distinction between permanent effects and temporary or transient effects. Permanent damage includes either a rearrangement of the atoms of the crystal lattice, such as, for example, the formation of vacancies, interstitials, or dislocations, or the introduction of foreign atoms into the crystal lattice as by transmutations. Transient effects result when electrons or holes are excited into nonequilibrium states. As a practical matter, transient effects have become an important problem for electronic circuits which must operate in an intense nuclear radiation environment. The largest class of these applications are in military electronics which potentially should function near a nuclear detonation.

An overall effect of high energy radiation is the emission of secondary electrons from materials. In a typical piece of electronic equipment, this effect will be exceeded only by that of carrier generation in semiconductors as a primary cause of transient radiation effects. The effects in semiconductors are usually overcome by avoiding the use of transistors and semiconductor diodes in such systems; however, such other common components in electronic systems as resistors, cables, insulators, low-value capacitors, and metal chassis parts give rise to spurious signals by emission or absorption of secondary electrons. Inan insulator, ionizing radiation interacting with atomic electrons can impart sufficient energy for these electrons to overcome the potential barrier or work function at the surface of the material and also the attraction of the image charges as they leave the material. In metals and semiconductors, the net space charge left behind by the emitted electrons is compensated by available free electrons. Because the conductivity of an insulator may be 3,371,146 Patented Feb. 27, 1968 ice quite low, a compensating charge cannot arrive promptly. Hence, secondary electron emission may produce relatively high conductivity paths or shunts which would allow larger-than-normal leakage currents to flow. In metals, ionization produced by radiation is very rapidly neutralized by the conduction electrons; however, the emission of secondary electrons from metals is an important source of injected electron effects in nearby electronic components. In general, secondary emission or absorption causes an electronic component to act as a current source due to the displacement or absorption of charge.

In the past, a number of techniques have been employed to minimize spurious signals due to secondary emission. One was to keep the circuit small. In general, signals due to secondary emission are proportional to the area exposed to the radiation. This technique, however, has not proved to be altogether satisfactory since further reduction in size of many military electronic equipment is not always practical. Another technique was to provide a balanced arrangement'in the circuit. A differential amplifier was coupled to this with the use of a virtual ground. This technique has proved to be quite effective in reducing spurious signals but has the disadvantage of requiring additional circuitry. It should be noted that both of these techniques attack the problem indirectly, that is, both techniques seek to minimize spurious signals due to secondary emission or adsorption of electrons rather than minimizing secondary emission of electrons in the circuit.

It is therefore an object of this invention to provide a method of reducing secondary emission of electrons induced by radiation in electronic systems.

It is another object of the instant invention to provide a method of minimizing spurious signals in electronic equipment due to secondary emission and absorption which does not require either a reduction in size of the equipment or the addition of compensating circuitry.

It is a further object of the invention to provide a method of reducing the transient effects caused by secondary emission due to intense high energy radiation in electronic circuits which is inexpensive and effective in many military electronic equipment without modification.

According to the present invention, the foregoing and other objects are attained by surrounding the electronic circuit in which it is desired to reduce secondary emission and adsorption with a strongly electronegative gas.

The signal seen from an object when secondary emission occurs depends not only on how much charge is emitted, but also on how the charge moves away and on the capacity to ground seen by the object. The voltage signal, V, is related to the charge-emitted, Q, and the capacity to ground, C, by the following relation:

Consider a situation where the capacitance to ground is taken, for simplicity, to be in the form of a parallel plate capacitor, the object being one plate and the other plate being grounded. If the electrons emitted from the object do not move all the way to the ground plate, their contribution to the observed emission current will be cut down by a factor of x/d, where x is the mean range of the electrons and d is the plate spacing. The plate spacing or the distance, d, of the object to ground is determined by the particular geometry of the electronic equipment and may be considered to be a given value or a constant. This invention causes the mean range, x, of emitted secondary electrons to be minimized and generation of additional secondary electrons to be sharply attenuated thereby effectively reducing the transient effects and spurious signals in electronic systems due to radiation induced secondary emission. This is accomplished by filling the elec- 3 tronic systems, using any conventional means appropriate to the gas utilized and the electronic ystem being filled, with an electron attaching gas. The particular electron attaching gas must be highly electronegative so that it can readily acquire free electrons, and at the same time, it must remain substantially non-conducting even though it has acquired excess charge. Obviously, the gas must itself be a poor source of secondary electron emission due to ionizing radiation. The halogens are highly electronegative gases but are corrosive when moisture is present or the ambient temperature is elevated; therefore, the halogens do not provide a suitable environment for electronic equipment except under certain controlled conditions. The various oxides of nitrogen are also highly electronegative. In particular, one atmosphere nitrogen dioxide and dinitrogen tetroxide, which form a stable mixture in the gaseous state at low to moderate temperatures, have been found to reduce the secondary emission current by a factor of more than 50 While very little conductivity was induced in the gas. The oxides of nitrogen, however, must be used with the same precautions as the halogens due to their highly toxic and corrosive nature. Oxygen is also an electron attaching gas, but it has been found that, while secondary electron emission is reduced in an oxygen environment, conductivity in the gas caused an increased signal. Another class of gases which are efiective in the reduction of secondary emission of electrons are fluorinated hydrocarbons. These gases while being electronegative are also relatively inert. They may, therefore, be used in certain situations Where the corrosive halogens and nitrogen oxides cannot be.

It will be apparent that the embodiments disclosed are only exemplary and that various modifications can be made in practice Within the scope of the invention as defined in the appended claim.

I claim as my invention:

1. The method of reducing secondary emission of electrons in electronic equipment induced by high energy nuclear radiation by at least a factor of 50 thereby reducing spurious signals caused 'by the emission and absorption of secondary electrons comprising the step of surrounding said electronic equipment with a highly electronegative yet low conductivity gas, said gas consisting essentially of a mixture of nitrogen dioxide and dinitrogen tetroxide.

References Cited UNITED STATES PATENTS 8/1958 Berberich 17417 6/1960 Liao 174l7 X OTHER REFERENCES The Condensed Chemical Dictionary, Reinhold, New York, 1952, p. 804. 

